1. Field of the Invention
This invention relates to colour developer compositions used to develop images in pressure-sensitive carbonless paper copying systems. This invention also relates to colour developer sheets for carbonless paper copying systems and to multiple carbonless paper copying systems comprising such colour developer sheets.
2. Description of the Prior Art
Pressure-sensitive carbonless paper copying systems have captured a very large market on account of their convenience compared with carbon paper copying. Carbonless paper copying systems comprise a multiple set made up of a matched pair, or a plurality of matched pairs, of sheets, generally of paper, of which one is a donating sheet and the other is a receiving sheet. When written upon, the donating sheet is pressed into contact with the receiving sheet, and an image substantially identical to the writing develops on the receiving sheet.
This image development is brought about by the contact of a colourless dye precursor, i.e. a colour former, present on the donating sheet with a colour developer present on the receiving sheet. Generally, a solution of the colour former in a solvent is encapsulated in microcapsules and coated on to the back face of the donating sheet (the CB sheet), which will generally have an uncoated front face. Typical colour formers, often termed "leuco dyes", are, for example, triphenylmethanes (such as Crystal violet lactone), xanthenes (such as N-102 fluoran) and thiazines (such as benzoyl leucomethylene blue). The colour developer is generally coated on the front face of the receiving sheet (the CF sheet). When the CB and CF sheets are placed in contiguous relationship with the colour developer adjacent to the colour former and pressure, such as from a writing instrument, typewriter or the like, is applied, the microcapsules are crushed and the released colour former solution is adsorbed onto the CF sheet where it contacts the colour developer. The image on the CF sheet develops from a chemical transformation of the colour former to a coloured form, caused by the colour developer. In general, this chemical transformation entails an acid-base type reaction wherein the colour former changes from an uncharged colourless state to a positively charged coloured state.
Alternatively, the colour former microcapsules and the colour developer can be coated onto the same sheet, or the colour developer can be on the CB sheet and the colour former microcapsules can be on the CF sheet. For convenience, hereinafter, "CB sheet" refers to a sheet, generally a paper sheet, coated with the colour former solution in microcapsules, and "CF sheet" refers to a sheet, generally a paper sheet, coated with a colour developer.
Currently, two types of colour developers are in wide use. The first type comprises the well known phenolic resins, generally low molecular weight compounds obtained by polymerising formaldehyde or the like and a diphenolic compound. Their use has several drawbacks. Firstly, they must be subjected to a long and complicated milling process to produce resins of specified particle size distributions suitable for use in carbonless paper copying systems. Secondly, they can decompose, particularly during milling and on the coated sheet, to release formaldehyde, a lachrymator and irritant now implicated as a possible human carcinogen. Thirdly, phenolic resins give a yellowish background to the CF sheet. This results in a deeper background colour for the writing image, which is accordingly of poorer quality. Finally, phenolic resins do not develop high image densities because they are of limited solubility in the colour former solution. This again results in poor image quality.
Monomeric phenols, including Bisphenol A and like bisphenols, are also known in the art as colour developers and components of colour developer compositions.
The second widely used type of colour developer comprises acid-treated clays, and they too are not without their drawbacks. Firstly, since the colour developing activity of an acid treated clay depends on the extent to which exchangeable cations in the clay are replaced by hydrogen to form colour developer sites on the clay surface, the clay must be carefully treated with a strong mineral acid. Secondly, to ensure a good quality image, sufficient contact between the colour solution and the colour developer sites on the clay surface is needed. The problem of achieving adequate contact between the colour former and the acid clay is analogous to the problem of low image density which results from the limited solubility of phenolic resins in the colour former solution. Image quality is highly sensitive to the chemistry and morphology of an acid clay, yet these parameters are very difficult to control. Nevertheless, it is generally considered necessary in order to obtain good image quality that the colour developer should be constituted, at least in the greater part, by such an acid clay.
CF sheets are prepared by coating a suspension of the colour developer and conventional coating additives such as adhesives, dispersants, protective colloids and the like, on to a sheet and letting the coating dry. High solids concentrations, which include high solids contents of the colour developer and any additive in the coating colour, are desirable for good image quality. However, concentrations above about 45-50% by weight are generally unattainable with phenolic resins or acid clays used alone or in predominant amounts as the colour developer, because of the poor rheology, or high viscosity, of coating compositions containing them.
It has been found that the rheological properties of a coating composition for a CF sheet may be improved by diluting or extending the colour developer with a substantially electrochemically inactive inorganic material such as a kaolinitic clay, a calcined kaolinitic clay, a natural or precipitated calcium carbonate, a natural or synthetic calcium sulphate or a talc. Typical coating compositions comprise from about 40% to about 98% by weight of the extender and from about 2% to about 60% by weight of the colour developer which may be an acid clay, a phenolic resin, a monomeric phenol, or a combination of any two or of all three of these types.
In addition to the requirement that a combination of a CB sheet and a CF sheet should be capable of producing an image of good quality when pressed upon by a writing, typewriting or printing instrument, it is also generally necessary for the sheets to be capable of receiving a good print image when printed by a conventional printing process, such as the web offset process or the sheet offset process. This property is necessary because the combination of the CB sheet and the CF sheet is generally required to bear verbal and/or graphic elements of a standard form.
Difficulties have been experienced in printing CF sheets, or webs of CF sheet material, by the offset process in that the printing ink is absorbed quickly into the sheet and dries undesirably rapidly, with the result that the ink becomes tacky and tends to adhere to the blanket of the offset printing press and accumulate thereon. This accumulation of highly tackified ink can also lead to removal of coating from the paper to create yet greater accumulation of material on the blanket. This problem is believed to be due to the structure of the CF sheet, which is deliberately made to be of high porosity in order to encourage good wetting of the colour developer by the solvent of the colour former, and by the strong adherence of the tacky ink to the coating surface. The accumulation of tacky ink on the blanket of an offset printing press is often known in the printing industry as "ink piling", and the removal by the tacky ink of coating pigment from the coated paper is known as "picking".
International Patent Application No. WO-93/09289 concerns a paper coating composition for preparing a coated paper for use in a gravure printing process, which paper coating composition contains a particulate inorganic paper-coating pigment which has been modified by treatment with a treating agent which renders the pigment surfaces hydrophobic or enhances their hydrophobicity. The treating agent has a non-polar hydrophobic portion comprising at least one hydrocarbon group having a chain length of from 8 to 30 carbon atoms and a polar portion which is capable of binding with sites on the pigment surfaces. The quantity of the treating agent used is from 0.05% to 5.0%, preferably from 0.1% to 2.0% by weight, based on the weight of the pigment. Evidence is given that the use of the coating composition makes it possible to prepare a coated paper which gives improved gravure printing results, and especially improved gravure print quality, print gloss and print density.
Thus, International Patent Application No. WO-93/09289 discloses that the treatment, with a treating agent of the type described, of a pigment of a paper coating composition will render the surfaces of the pigment hydrophobic and will enhance certain of the gravure printing properties of a coated paper prepared from the composition.